Robert H. Clifford

Investigation of a high-efficiency nebulizer and a thimble glass frit nebulizer for elemental analysis of biological materials by inductively coupled plasma-atomic emission spectrometry

Abstract A high-efficiency nebulizer (HEN) and a demountable thimble glass frit nebulizer are evaluated for argon inductively coupled plasma-atomic emission spectrometry (Ar ICP-AES). Aerosol characteristics for the HEN and the thimble frit are studied using laser Fraunhofer diffraction. Analyte transport efficiencies, mass transport rates, detection limits, and short- and long-term stabilities are measured. Four standard reference materials (trace elements in water, oyster tissue, bovine liver, and orchard leaves) are analyzed. For comparison, results obtained by a high-solid pneumatic nebulizer (conespray nebulizer) are also presented. Transport efficiencies of up to 50–60% are obtained for the HEN and the thimble frit nebulizer at an uptake rate of 11 μl min. For the elements tested, short- and long-term precisions and detection limits obtained with the thimble frit are comparable to those of a commonly used pneumatic nebulizer. Good agreement with certified values is obtained for six trace elements in a standard reference material (SRM) water sample nebulized by the thimble frit. However, signal suppression is observed when the biological or plant SRMs are nebulized with the thimble frit nebulizer. In contrast, satisfactory results are obtained for the same matrices when the HEN and the conespray nebulizer are used.

Diagnostic studies on desolvated aerosols from ultrasonic nebulizers

Abstract Dual-beam, light-scattering interferometry was used for simultaneous measurements of particle-size and particle-velocity distributions, size-velocity correlation, particle number density, and volume flux and span of desolvated aerosols. A commercial ultrasonic nebulizer (USN) and a low-cost, humidifier-based USN were used to nebulize pure water, or aqueous solutions containing H 2 SO 4 , HNO 3 , or NaCl. In general, Sauter mean diameter, velocity of droplets, and volume flux of desolvated aerosol were larger when Ar was used as the injector gas instead of He. Sauter mean diameter increased with acid concentration, but it was independent of salt concentration. The velocity of desolvated droplets did not change with analyte concentration or the temperature of the heating tube. Pulsations and clustering of particles were observed for the first time by time-resolved measurements on the aerosol before injection into an inductively coupled plasma. In certain cases, the local number density of the desolvated particles within the clusters and the diameter of the dry particles varied by a factor of ten on a millisecond time scale. The implications of these observations in plasma spectrochemical measurements are discussed.

Particle size measurements in the submicron range by the differential electromobility technique: comparison of aerosols from thermospray, ultrasonic, pneumatic and frit-type nebulizers

Abstract The differential electromobility technique was used for the comparison of droplet- and particle-size distributions in the 0.02–0.8 μm range for six nebulization systems often used in inductively coupled plasma (ICP) spectrometry: a thermospray nebulizer coupled to a membrane separator (TNMS); two ultrasonic nebulizers (USNs) used with desolvation; one pneumatic nebulizer (PN) used with and without desolvation; and a frit-type nebulizer. In general, volume concentration (volume of droplets or particles per cubic centimeter of injector gas) increased with NaCl concentration, and it was greater for TNMS followed by USNs compared to other nebulizers. For the desolvated aerosol produced by PN and USN, volume concentration was found to be independent of the temperature (140–180°C) of the heating tube for the desolvation device. As the nebulizer tip temperature in thermospray nebulization was varied from 160 to 240°C, a larger volume concentration of desolvated aerosol was produced. Size distributions shifted towards larger particles with increasing NaCl concentration. The implications of these observations in plasma spectrochemical measurements are discussed.

Noise power spectra comparing He and Ar ICP discharges

Abstract Noise power spectra (NFS) of helium inductively coupled plasmas (He ICPs) were measured using tangential- and laminar-flow torches. To identify the noise sources originating from the sample introduction systems and the plasmas, NPS were measured for two frequency ranges: 0–35 and 0–1060 Hz. Spectra were compared with those obtained for the commonly used Ar ICP. Examination of the NPS reveals and reconfirms that: (a) the He ICP generated in a tangential-flow torch experiences a greater level of high-frequency fluctuation than the Ar ICP, (b) no high-frequency peaks are observed when a laminar-flow torch is used to form the He ICP, and (c) the major contribution to noise at low frequency results from the injection of aqueous samples into the plasma. Results are discussed in terms of plasma asymmetries, vortex ring phenomenon, and instabilities introduced by pneumatic and ultrasonic nebulizers.

Atomic emission spectrometric detection limits and noise power spectra of argon inductively coupled plasma discharges formed with laminar- and tangential-flow torches

Atomic emission spectrometric detection limits and noise power spectra were measured for argon inductively coupled plasmas (Ar ICPs) formed in tangential- and laminar-flow torches. To identify the origin of the noise sources from the sample introduction systems and the plasmas, noise power spectra were collected for two frequency ranges: 0–35, and 0–1060 Hz. In contrast to previous reports, the replacement of tangential flow by laminar flow resulted in no significant improvement in the detection power of the Ar ICP when torches with extended outer tubes were used.

Fundamental properties of helium inductively coupled plasmas measured by high-resolution Fourier transform spectrometry

Abstract Intensities and widths of atomic spectral lines of He, H, and Fe, excited in an atmospheric-pressure helium inductively coupled plasma (He ICP) were measured with a high-resolution Fourier transform spectrometer. These data along with measured intensities of rotational bands, such as B 2 Σ + u → X 2 Σ + g of N + 2 and A 2 Σ + → X 2 Π i of OH, were used to estimate excitation, rotational, and Doppler temperatures, and electron number densities. Similar to an Ar ICP, the widths of Fe lines ranged from 3 to 4 pm. The line widths of H and He lines in the spectra from a dry He ICP generally were larger than those observed from a wet plasma. The line-width data for H were used to evaluate electron number densities in wet and dry He ICP discharges. The excitation temperature of He ICP was dependent upon the energy levels of the selected thermometric species. The rotational temperatures measured from OH (3000 K) and N + 2 (2200 K) were substantially different from those of an Ar ICP. The presence of water in the aerosol resulted in an increase in the excitation temperature of the He ICP, similar to the trend observed earlier for the Ar ICP. The implications of these results in He ICP spectrochemical analysis are discussed